Re: Can you solve this function chaining quiz?

From:

DeMarcus <use_my_alias_here@hotmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Fri, 21 May 2010 06:32:32 CST

Message-ID:

<4bf65369$0$282$14726298@news.sunsite.dk>

On 2010-05-21 05:34, Michael Herstine <sp1ff@pobox.com> wrote:

I have a problem. I want to throw ExceptionA derived from Exception, i.e.

struct ExceptionA : Exception
{

};

ExceptionA, ExceptionB, ExceptionC, etc. shall be able to attach
text to the exception directly at construction via function
chaining, i.e.

throw ExceptionA().addText( "Hello" ).addText( "World" );

The quick way to implement that would be the following.

struct Exception
{
     Exception& addText( const std::string& str )
     {
        // Add str to some private string.
        return *this;
     }

};

Now, the problem comes with the throw.

throw ExceptionA().addText( "Hello" ).addText( "World" );

That will slice the exception and not throw an ExceptionA but
instead just a plain Exception since that is what addText()
returns.

I do not entirely understand why this is necessary. If this addText
function is proving difficult, why not do something more obvious?

      std::string exceptionText = "Hello ";
      exceptionText += "world";
      throw ExceptionA( exceptionText ); // Make ctor accept string.

Maybe this is overly simplistic. Maybe you want something to be done
to the string, other than concatenation. In that case, write a
function that concatenates the string with whatever extra you need!

I'm sorry if I was a bit unclear, but with the function chaining I want
to add more functions like addErrno(), addInnerException(),
addWhatever(). So the problem still remains.

[snip]

Is part of the specification for the problem "Must be done in one
line."? All this seems like a lot of work for just pretty syntax,
unless i'm missing something.

Yes, you are completely right, that is why I brought this question here.
Is it possible to make the syntax even better without drawbacks? I.e.

throw ExceptionA().addText("x").addText("y");

Maybe I'm not fully appreciating the problem, here, but it seems to me
you want to succinctly "attach" data to the exception in a way
expressable through interface (as oppossed to boost::exception). Why
not just use the constructor?

Using the constructor was my first attempt. However, lately I've started
to appreciate function chaining, especially for optional parameters
where the number of parameters may grow.

Having n optional parameters in the constructor will give you 2^n
constructors unless you use pointers that can be null. Not using
pointers will also get you into trouble if two parameters are of the
same type. Therefore, in this case, I found function chaining more
appealing than putting the parameters in the constructor.

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